Derived metrics provide a way of extending the Performance Metrics Name Space (PMNS) with new metrics defined at the PCP client-side using arithmetic expressions over the existing performance metrics.

Typical uses would be to aggregate a number of similar metrics to provide a higher-level summary metric or to support the “delta V over delta V” class of metrics that are not possible in the base data semantics of PCP. An example of the latter class would be the average I/O size, defined as delta(disk.dev.total_bytes) / delta(disk.dev.total) where both of the disk.dev metrics are counters, and what is required is to to sample both metrics, compute the difference between the current and previous values and then calculate the ratio of these differences.

The arguments to pmRegisterDerived are the name of the new derived metric and expr is an arithmetic expression defining how the values of name should be computed.

pmRegisterDerivedMetric is the exact functional equivalent to pmRegisterDerived except that it provides a simplified model of error handling, where a formatted message is returned via the errmsg parameter.

Syntactic checking is performed at the time pmRegisterDerived is called, but semantic checking is deferred until each new context is created with pmNewContext(3) or re-established with pmReconnectContext(3), at which time the PMNS and metadata is available to allow semantic checking and the metadata of the derived metrics to be established. This means pmRegisterDerived does not apply retrospectively to any open contexts, so the normal use would be to make all calls to pmRegisterDerived (possibly via pmLoadDerivedConfig(3)) or pmRegisterDerivedMetric and then call pmNewContext(3).

name should follow the syntactic rules for the names of performance metrics, namely one or more components separated with a dot (“.”), and each component must begin with an alphabetic followed by zero or more characters drawn from the alphabetics, numerics and underscore (“_”). For more details, refer to PCPIntro(1) and pmns(5).

name must be unique across all derived metrics and should not match the name of any regular metric in the PMNS. It is acceptable for name to share some part of its prefix with an existing subtree of the PMNS, e.g. the average I/O size metric above could be named disk.dev.avgsz which would place it amongst the other disk.dev metrics in the PMNS. Alternatively, derived metrics could populate their own subtree of the PMNS, e.g. the average I/O size metric above could be named my.summary.disk.avgsz.

The expression expr follows these syntactic rules:

*

Terminal elements are either names of existing metrics or integer constants. Recursive definitions are not allowed, so only the names of regular metrics (not other derived metrics) may be used. Integer constants are constrained to the precision of 32-bit unsigned integers.

*

The usual binary arithmetic operators are supported, namely - addition (“+”), subtraction (“-”), multiplication (“*”) and division (“/”) with the normal precedence rules where multiplication and division have higher precedence than addition and subtraction, so a+b*c is evaluated as a+(b*c)

*

Parenthesis may be used for grouping.

*

The following unary functions operate on a single performance metric and return one or more values. For all functions (except count() and instant()), the type of the operand metric must be arithmetic (integer of various sizes and signedness, float or double).

Function

Value

avg(x)

A singular instance being the average value across all instances for the metric x.

count(x)

A singular instance being the count of the number of instances for the metric x.

delta(x)

Returns the difference in values for the metric x between one call to pmFetch(3) and the next. There is one value in the result for each instance that appears in both the current and the previous sample.

rate(x)

Returns the difference in values for the metric x between one call to pmFetch(3) and the next divided by the elapsed time between the calls to pmFetch(3). The semantics of the derived metric are based on the semantics of the operand (x) with the dimension in the time domain decreased by one and scaling if required in the time utilization case where the operand is in units of time, and the derived metric is unitless. This mimics the rate conversion applied to counter metrics by tools such as pmval(1), pmie(1) and pmchart(1). There is one value in the result for each instance that appears in both the current and the previous sample.

instant(x)

Returns the current value of the metric x, even it has the semantics of a counter, i.e. PM_SEM_COUNTER. The semantics of the derived metric are based on the semantics of the operand (x); if x has semantics PM_SEM_COUNTER, the semantics of instant(x) is PM_SEM_INSTANT, otherwise the semantics of the derived metric is the same as the semantics of the metric x.

max(x)

A singular instance being the maximum value across all instances for the metric x.

min(x)

A singular instance being the minimum value across all instances for the metric x.

sum(x)

A singular instance being the sum of the values across all instances for the metric x.

There are a number of conversions required to determine the metadata for a derived metric and to ensure the semantics of the expressions are sound.

In a binary expression, if the semantics of both operands is not a counter (i.e. PM_SEM_INSTANT or PM_SEM_DISCRETE) then the result will have semantics PM_SEM_INSTANT unless both operands are PM_SEM_DISCRETE in which case the result is also PM_SEM_DISCRETE.

The mapping of the pmUnits of the metadata uses the following rules:

*

If both operands have a dimension of COUNT and the scales are not the same, use the larger scale and convert the values of the operand with the smaller scale.

*

If both operands have a dimension of TIME and the scales are not the same, use the larger scale and convert the values of the operand with the smaller scale.

*

If both operands have a dimension of SPACE and the scales are not the same, use the larger scale and convert the values of the operand with the smaller scale.

*

For addition and subtraction all dimensions for each of the operands and result are identical.

*

For multiplication, the dimensions of the result are the sum of the dimensions of the operands.

*

For division, the dimensions of the result are the difference of the dimensions of the operands.

Scale conversion involves division if the dimension is positive else multiplication if the dimension is negative. If scale conversion is applied to either of the operands, the result is promoted to type PM_TYPE_DOUBLE.

Putting all of this together in an example, consider the derived metric defined as follows:x = network.interface.speed - delta(network.interface.in.bytes) / delta(sample.milliseconds)The type, dimension and scale settings would propagate up the expression tree as follows.

Expression

Type

Dimension & Scale

Scale Factor(s)

sample.milliseconds

DOUBLE

millisec

delta(...)

DOUBLE

millisec

network...bytes

U64

byte

delta(...)

U64

byte

delta(...) / delta(...)

DOUBLE

byte/millisec

/1048576 and *1000

network...speed

FLOAT

Mbyte/sec

x

DOUBLE

Mbyte/sec

Because semantic checking cannot be done at the time pmRegisterDerived is called, errors found during semantic checking are reported using pmprintf(3). These include:

Error: derived metric <name1>: operand: <name2>: <reason>

There was a problem calling pmLookupName(3) to identify the operand metric <name2> used in the definition of the derived metric <name1>.

Error: derived metric <name1>: operand (<name2> [<pmid2>]): <reason>

There was a problem calling pmLookupDesc(3) to identify the operand metric <name2> with PMID <pmid2> used in the definition of the derived metric <name1>.

Rate conversion using the rate() function is only possible for operand metrics with a Time dimension of 0 or 1 (see pmLookupDesc(3)). If the operand metric's Time dimension is 0, then the derived metrics has a value "per second" (Time dimension of -1). If the operand metric's Time dimension is 1, then the derived metrics has a value of time utilization (Time dimension of 0).

For the binary arithmetic operators, if either operand must be scaled (e.g. convert bytes to Kbytes) then the result is promoted to PM_TYPE_DOUBLE. Otherwise the type of the result is determined by the types of the operands, as per the following table which is evaluated from top to bottom until a match is found.

If a syntactic error is found at the time of registration, the value returned by pmRegisterDerived is a pointer into expr indicating where the error was found. To identify what the error was, the application should call pmDerivedErrStr(3) to retrieve the corresponding parser error message.

pmRegisterDerivedMetric returns 0 and errmsg is undefined if the parsing is successful.

If the given expr does not conform to the required syntax pmRegisterDerivedMetric returns -1 and a dynamically allocated error message string in errmsg. The error message is terminated with a newline and includes both the input name and expr, along with an indicator of the position at which the error was detected. e.g.Error: pmRegisterDerivedMetric("my.disk.rates", ...) syntax error

The position indicator line may be followed by an additional diagnostic line describing the nature of the error, when available.

In the case of an error, the caller is responsible for calling free(3) to release the space allocated for errmsg.